The methods and corresponding equipment currently in use for these purposes involve suspension of the thruster on the balance. FIGS. 1 and 2 show two embodiments of conventional equipment. Both cases involve measuring equipment which uses two arms which rotate around a fulcrum F. The thruster P is disposed on one arm C, whereas the measurable balancing force acts on the other arm B.
In the known solution shown schematically in FIG. 1, in addition to the arms B and C there are shown: a cable A which serves the dual purpose of supporting the entire structure, and of acting as a torsional spring; the point of rotation F of the structure, which is prevented by the cable A from being displaced at all; the electromagnet D which counterbalances the thrust of the thruster P; and the position feed-back system E which enables the angular displacements to be reset by means of an optical system for measuring the displacements, comprising a mirror G. The measurement is thus performed on the horizontal plane and the balancing force is of an electromagnetic nature. Since the length of the arms B and C is known, the magnitude of the balancing force permits determination of the thrust T exerted by the thruster P on the structure of the balance.
In the embodiment shown schematically in FIG. 2, in addition to the arms B and C there are shown the center of rotation F of the structure, and the load cell D. The operation is similar to that described with reference to the equipment in FIG. 1.
The limits of the conventional equipment of the above-described type are derived from the fact that although the balance must measure thrusts T of approximately 1 mN to 100 mN, it must support the weight of the entire thruster, which is of the order of kilograms. In addition the friction exerted on the balance can become a force comparable to the thrusts to be measured, since the thruster P is connected to the supply system by means of electrical power cables and ducts for the propellant gas supply. These elements introduce into the system forces and damping which are difficult to quantify and reproduce. Considerable problems are caused in these test configurations by two elements, i.e. the fulcrum of the balance, and the power cables: in order for the balance to operate perfectly, the fulcrum must not give rise to friction or develop unknown forces with components in the direction of the thrust to be measured. Both requirements are difficult to obtain in equipment of the conventional type, particularly taking into account the fact that the cables constitute constraints relative to the exterior, with damping and rigidity which are difficult to reproduce exactly from one test to another.